Cytomegaloviruses (CMV's) are ubiquitous in nature and are the most common causes of intrauterine infection. Congenital infection is common in newborns of infected mothers. In some populations, as much as 10% of children display perinatal infections. In a small percentage of newborns, the infection is virulent, involving multiple organs. Pronounced involvement of the reticuloendothelial and central nervous system is typical; and the infection is a major cause of mental retardation. Careful testing demonstrates that as many as 50% of severely, prenatally infected adults may display neuropsychiatric disease or deafness. Although extraneural organs are usually spared chronic morbidity, the virus can be detected in the kidney for years.
In the adult, cytomegalovirus-induced mononucleosis is a lingering illness that causes significant morbidity. If it occurs in immunosuppressed patients, the disease is more severe, and it may be complicated by other infectious pathogens which may be fatal. Cytomegalovirus retinitis is a severe problem in immunosuppressed patients that often leads to blindness. Immunosuppressed patients are also very susceptible to CMV pneumonitis, which is one of the most lethal of human viral diseases. Although cytomegalovirus may play a role in the progression of HIV infection to AIDS by stimulating the transcription of the HIV long terminal repeats (LTR) in nontransformed co-infected T cells, histologic examination of adrenals and brains from AIDS patients has suggested that the adrenalitis, encephalitis and peripheral neuropathy were caused by CMV infection.
CMV is considered to be an oncogenic virus. In vitro, CMV can transform cells and stimulate growth. Both human and non-human cells can undergo transformation when incubated with CMV. Transformed cells contain CMV antigens that are oncogenic when inoculated into appropriate animals. Moreover, oncogenic potential has been associated with specific segments of the CMV genome.
Human CMV is a large, enveloped herpesvirus whose genome consists of a double-stranded DNA molecule which is approximately 240,000 nucleotides in length. This genome is the most complex of all DNA viruses and is approximately 50% larger than the genome of herpes simplex virus (HSV). Intact viral DNA is composed of contiguous long (L) and short (S) segments, each of which contains regions of unique DNA sequence flanked by homologous regions of repetitive sequence. As a group, the human CMV isolates share at least 80% sequence homology, making it nearly impossible to classify cytomegaloviruses into subgroups or subtypes, although variations in the restriction endonuclease patterns of various CMV DNA preparations are identifiable in epidemiologically unrelated strains. The DNA of the prototypic strain of CMV (AD 169) has been sequenced and reported to contain a conservative estimate of 175 unique translational open reading frames (ORFs). A number of the predicted CMV gene products show homology to other human herpesvirus gene products. At least 42 ORFs encode putative glycoproteins and several of the CMVORFs putatively encode proteins with amino acid homology to human opsin receptor proteins.
In permissive human fibroblasts, CMV gene expression is regulated by a cascade of genetic events that act at both the transcriptional and translational levels. CMV gene expression can be divided into three phases which resemble those of HSV defined as the immediate early (IE), early and late periods. Following adsorption, penetration and uncoating of the virus, a group of viral transcripts, immediate early messenger RNAs (IE mRNAs) are synthesized within 1-4 hours even in the presence of translational inhibitors such as cycloheximide. In the normal course of infection, the IE mRNAs are translated and their protein products are instrumental in the onset of early transcriptional events. At least 4 proteins are synthesized from IE mRNAs; of these, one is a glycoprotein. The IE1 and IE2 proteins are transcriptional activating factors for other CMV genes and the IE3 protein encompasses a region of the CMV genome which can transform NIH 3T3 cells in vitro. Early proteins are encoded by the mRNAs which are synthesized prior to viral DNA synthesis. A number of the early proteins play a role in nucleotide metabolism and DNA synthesis in the infected cell. After the onset of viral DNA synthesis, the transcription of the late mRNAs is maximal and probably reflects a template abundancy requirement similar to that observed for analogous HSV mRNAs. The late CMV proteins include the glycoprotein constituents of the viral envelope, the viral capsid proteins and other proteins which are necessary for assembly or structural integrity of the mature CMV particle and/or egress of the assembled virion from the infected cell. In addition to the transcriptional controls operant upon CMV gene expression, examples of post-transcriptional controls are known to influence the appearance of some CMV proteins. Splicing of mRNAs is more common than observed in HSV gene expression and the nucleotide sequence composition of the 5' nontranslated region in the cognate mRNA is reported to influence the synthesis of at least one early CMV protein.
Effective therapy for CMV has not yet been developed despite studies on a number of antivirals. Interferon, transfer factor, adenine arabinoside (Ara-A), acycloguanosine (Acyclovir, ACV) and certain combinations of these drugs have been ineffective in controlling CMV infection. Based on preclinical and clinical data, foscarnet (PFA) and ganciclovir (DHPG) show limited potential as antiviral agents. PFA treatment has resulted in the resolution of CMV retinitis in five AIDS patients. DHPG studies have shown efficacy against CMV retinitis or colitis. DHPG seems to be well tolerated by treated individuals, but the appearance of a reversible neutropenia, the emergence of resistant strains of CMV upon long-term administration, and the lack of efficacy against CMV pneumonitis limit the long term applications of this compound. The development of more effective and less-toxic therapeutic compounds and methods is needed for both acute and chronic use.
Classical therapeutics has generally focused upon interactions with proteins in efforts to moderate their disease causing or disease potentiating functions. Such therapeutic approaches have failed for cytomegalovirus infections. The present invention is directed to an alternative approach to the treatment of such infections, the antisense inhibition of cytomegalovirus gene expression through the mediation of oligonucleotides.
Antisense methodology is the complementary hybridization of relatively short oligonucleotides to single-stranded mRNA or single-stranded DNA, or even double stranded DNA, such that the normal, essential functions of these intracellular nucleic acids are disrupted. Hybridization is the sequence specific hydrogen bonding of oligonucleotides to Watson-Crick base pairs of RNA or single stranded DNA. Such base pairs are said to be complementary to one another.
The events which disrupt nucleic acid function are discussed by Cohen in Oligonucleotides:Antisense Inhibitors of Gene Expression, CRC Press, Boca Raton Fla., 1989, who proposes two possible types of terminating events. The first, hybridization arrest, denotes a terminating event in which the oligonucleotide inhibitor binds to the target nucleic acid and thus prevents, by simple steric hindrance, the binding of essential proteins, most often ribosomes, to the nucleic acid. Methyl phosphonate oligonucleotides; P. S. Miller & P.O.P. Ts'O, Anti-Cancer Drug Design 1987, Vol. 2, pp. 117-128; and .alpha.-anomer oligonucleotides, Cohen J. S. ed., Oligonucleotides: Antisense Inhibitors of Gene Expression, CRC Press, Boca Raton Fla., 1989, are two of the most extensively studied antisense agents which are thought to disrupt nucleic acid function by hybridization arrest.
A second type of terminating event for antisense oligonucleotides involves enzymatic cleavage of the targeted RNA by intracellularRNase H. The oligonucleotide, which must be of the deoxyribo type, hybridizes with the targeted RNA and this duplex activates the RNase H enzyme to cleave the RNA strand, thus destroying the normal function of the RNA. Phosphorothioate oligonucleotides are a prominent example of an antisense agent which operates by this type of terminating event.
Considerable research is being directed to the application of oligonucleotides as antisense agents for therapeutic purposes. Applications of oligonucleotides as diagnostics, research reagents, and potential therapeutic agents require that the oligonucleotides be synthesized in large quantities, be transported across cell membranes or taken up by cells, appropriately hybridize to targeted RNA or DNA, and subsequently terminate or disrupt nucleic acid function. These critical functions depend on the initial stability of oligonucleotides towards nuclease degradation.
Oligonucleotides modified to exhibit resistance to nucleases, to activate the RNase H terminating event, and to hybridize with appropriate strength and fidelity to targeted RNA (or DNA) are greatly desired for antisense oligonucleotide diagnostics, therapeutics and research with cytomegaloviruses.